1. Field of the Invention
This invention relates to optically phase-locked semiconductor microwave diodes, IMPATT diodes in particular.
2. Description of the Prior Art
IMPATT diodes are currently the best high-power solid-state microwave sources. An IMPATT diode consists of an active region comprising an avalanche region and an adjacent drift region. When a dc avalanching reverse bias and a microwave voltage are applied across a rectifying barrier, such as a Schottky barrier, adjacent to the avalanche region, carriers are generated in the avalanche region and, moving across the drift region, create a current which is 180.degree. out of phase with the applied microwave voltage. When placed in an appropriate cavity, this negative resistance sustains oscillation. The thin multi-layer active region is normally placed upon a thicker substrate for mechanical strength.
IMPATT diodes suffer from thermal instability and frequency chirping in the free-running mode, and external stabilization must therefore be provided. Electrical injection locking has been the technique most widely used to provide such stabilization. Electrical stabilization, however, requires high-power, high-frequency stable drivers to provide a stable millimeter wave signal. These can only be realized by cascade frequency miltiplication and power amplification and therefore require complicated, bulky, heavy microwave circuits.
A potentially superior method of phase-locking is optical injection locking in which a reference oscillator modulates the amplitude of an optical beam that illuminates the active region of the IMPATT. Optical injection locking can provide greater isolation than electronic stabilization and with it effective subharmonic locking can be achieved. Principally, however, it can be lighter, more compact and more flexible than electronic injection locking because the microwave wave guides can be replaced by optical fibers for injecting the reference signal.
A major problem with optical injection locking has been low optical coupling efficiencies due to poor light coupling into the active region of the IMPATT. The thinness of the active region layers (on the order of one micrometer) makes it difficult to confine the light to that region when irradiating from the edge. If light is injected through the outer support layer toward the active region, much of it is generally absorbed by the thick outer layer. Another problem is the difficulty of controlling the location of light absorption and its associated carrier generation (electrons and holes). Since an IMPATT diode that consists of several layers is a transit time device and since carriers generated indifferent layers require different transit times to reach the avalanche region and to leave the active region, controlling the location of carrier generation is important for optical injection locking.